Python manage.py migrate 时。总时出现这个 warnings 是什么意思？

[python30]

C:\Users\MrTong\Desktop\django\EasyCRM>Python manage.py migrate
System check identified some issues:
WARNINGS:
crm.Customer.tags: (fields.W340) null has no effect on ManyToManyField.
crm.UserProfile.roles: (fields.W340) null has no effect on ManyToManyField.

[georgema1982]

你在 ManyToManyField 上有个 null 的参数。migration 提醒你 null 在 ManyToManyField 上无效

[python30]

多谢。